1. Field of the Invention
In a boiling water nuclear reactor, a fissile fuel atom, such as U235, PU239, or PU241 absorbs a neutron in its nucleus and undergoes nuclear disintegration. This produces on the average two fissile fragments of lower atomic weight with great kinetic energy and several neutrons, these neutrons also being at high energy.
In the boiling water nuclear reactor, the nuclear fuel is in the form of fuel rods, each of which comprises a plurality of scintered pellets contained within an elongate sealed cladding tube or "fuel rod." Groups of such fuel rods are supported between upper and lower tie plates to form separately replaceable fuel assembles or bundles. A sufficient number of such fuel assemblies are arranged in a matrix, approximating a right circular cylinder, to form the nuclear reactor core which is capable of self-sustained fission reaction. The kinetic energy of the fission products is dissipated as heat in the fuel rods. Energy is also deposited in the fuel structure and moderator by the neutrons, gamma rays and other radiation resulting from the fission process. The core is submerged in coolant which removes the heat. Typically such heat removal occurs by the coolant water boiling into steam. From the steam energy is extracted to perform useful work.
In a boiling water nuclear reactor the coolant also acts as a neutron moderator. This moderator takes the emitted high energy neutrons and slows down the neutrons to render them thermal in character and hence more likely to be absorbed in the fuel continuing the fission reaction.
The commonly used fuel for water cooled and moderated nuclear power reactors comprises uranium dioxide of which from about 0.7 to 5.0% is fissile U235 mixed with fertile U238. During operation of the reactor, some of the fertile U238 is converted to fissile plutonium PU-239 and PU-241. It turns out that the U238 is also fissionable, but only for high energy neutrons. The ratio of fissile materials produced (for example plutonium 239 and plutonium 241) to fissile material destroyed (for example U235, plutonium 239 and plutonium 241) is defined as the "conversion ratio".
Fuel bundles are typically replaced at certain "outages". Typically these outages delineate "cycles". During such outages the reactor is opened and remote lifting equipment removes fuel bundles with spent fuel therein and replaces the fuel bundles with those having fresh fissile materials contained within the fuel rods. At each outage, only a portion of the total fuel bundles are removed. This portion is in the order of 25%.